FIG. 5 shows a basic structure of a wavelength-division multiplexed optical transmission network. In wavelength-division multiplexed optical transmission networks, in general, since optical signals of a plurality of adjacent wavelengths are transmitted in a wavelength-division multiplexed form, they need a reference light used as a reference of optical frequencies. Connected to an optical network 10 are a plurality of communication stations 12-1 through 12-7 via optical branching lines 14-1 through 14-7. An optical frequency reference device for generating an optical frequency reference light is provided in the communication station 12-1. The optical frequency reference light output from the communication station 12-1 is sent to the optical network 10 via the optical branching line 14-1, and further to other communication stations 12-2 to 12-7 through the optical branching lines 14-2 to 14-7. From the viewpoint that the communication station 12-1 generates the optical frequency reference light, it is the reference station, and the other communication stations 12-2 through 12-7 are slave stations from the viewpoint that they use the optical frequency reference light from the exterior station 12-1, i.e. the reference station, to stabilize optical frequencies of their own transmission laser devices.
FIG. 6 is a schematic block diagram showing a general construction of a conventional slave station. Its details are discussed by M. W. Maeda, et al. in "Absolute Frequency Identification and Stabilization of DFB Lasers in 1.54 .mu.m Region", Electronics Letters, Vol. 25, No. 1, pp. 9-11, 1989.
Numerals 20-1, 20-2 and 20-3 denote transmission laser devices for generating transmission optical signals having different wavelengths each other. Their optical outputs and the optical frequency reference light from the network 10 are multiplexed by an optical multiplexer 22 and applied to an optical frequency discriminator 24. Typically used as the optical frequency discriminator 24 is a scanning Fabry-Perot interferometer which converts the optical frequency domain to a time domain by scanning. The optical frequency discriminator 24 repeatedly scans optical outputs of the optical multiplexer 22 over a bandwidth on the optical frequency domain in response to a sweep signal from a control device 26. A photodetector 28 converts an optical output of the optical frequency discriminator 24 into an electric signal, and applies it to the control device 26. By scanning the optical frequency discriminator 24 and by measuring the output level of the photodetector 28 on the time domain, the spectrum of the optical output of the optical multiplexer 22 can be measured.
FIG. 7 shows an example of spectral measurement by the optical frequency discriminator 24. Time along the abscissa corresponds to optical frequencies by scanning. In FIG. 7, (1) is a sweep signal from the control device 26, and (2) is an output of the photodetector 28. Numeral 30 denotes optical frequency reference light from a reference station, and numerals 32-1, 32-2 and 32-3 are optical outputs of the transmission laser devices 20-1, 20-2 and 20-3, respectively. The control device 26 measures sweep times .DELTA.t1, .DELTA.t2 and .DELTA.t3 of the optical outputs 32-1, 32-2 and 32-3 of the transmission laser devices 20-1, 20-2 and 20-3 from the optical frequency reference light 30, and controls drive currents and operation temperatures of the transmission laser devices 20-1, 20-2 and 20-3 so as to maintain the sweep times .DELTA.t1, .DELTA.t2 and .DELTA.t3 at predetermined values. In this manner, optical frequencies of optical outputs of the transmission laser devices 20-1, 20-2 and 20-3 can be stabilized at predetermined values relative to the optical frequency of the optical frequency reference light from the reference station 12-1.
In the conventional frequency stabilizing system, if the optical frequency reference light from the reference station 12-1 is shut off, then the reference for the measurement of time intervals is lost, and behaviors for stabilizing optical frequencies are adversely affected. A possible approach to deal with the situation is to use optical output of one of transmission laser devices (for example, device 20-1) as a provisional optical frequency reference. However, it is not promised that the transmission laser device (e.g. the device 20-1) used as the provisional reference be always used for communication. The output of the device is possibly cut off when it is not used.
It will be possible to hold the output of one of the transmission laser devices used as the provisional optical frequency reference always ON. However, optical demand/assignment systems or optical ATM systems, for example, need the existence of a period of time where the output of the transmission laser device is OFF. Therefore, such systems result in loosing the provisional optical frequency reference during the period and fail in stabilizing the optical frequencies of other transmission optical signals.
When the optical output of the transmission laser device is modulated for data transmission, it is spread over a wide spectral range, which becomes a factor of an error upon determining the reference point on the time base. Therefore, the use of data-modulated transmission light as the provisional optical frequency reference is not desirable.